A Varactor Diode is a type of diode which is used as a capacitor. The varactor diode is a PN diode. The diode is connected in reverse bias. When the reverse bias voltage is applied, the depletion zone in the PN junction varies depending on the magnitude of the voltage.
No current flows through the diode, however the capacitance of the diode changes with the depletion zone in the PN junction. Varactor Diodes are usually used in Tuning Circuits. They have a variable capacitance as the capacitance value can be controlled by adjusting the voltage.
Varactor diodes are available in a wide range of capacitance values from 22pF to 33pF. Varactor Diodes are also known as Varicaps or variable capacitance Diodes or tuning Diodes